Up to now, scientists have only been able to assess the survival status of a relative handful of species due to the sheer variety of life forms inhabiting the planet.

The newly updated "Red List" -- widely viewed as the global standard for conservation monitoring -- includes assessments of 44,838 species, mostly mammals, birds, amphibians and some plants.

But this is only a tiny fraction of the world's life forms, which almost certainly numbers in the tens of million, many of them microorganisms.

"The status of the rest of the world's biodiversity is very poorly known," Jonathan Baillie, director of conservation programmes at the Zoological Society of London, said at the World Conservation Congress in Barcelona.

"The problem is the size of some of these groups -- how do you assess a million plus beetles?"

One way is to borrow a technique from opinion polls and stock market indices such as the Dow Jones or Nasdaq: a representative sampling.

"We have political, social and economic indices, but we lack broad biodiversity indices for the very things that underlie our existence," he said.

Scientists first tested the concept by assessing 1,500 randomly selected reptiles, much as survey institutes might poll a thousand likely voters before an election.

What they found was that about 22 percent of the world's reptiles could be in the process of dying out.

When that tally was added to what was already known about mammals, birds and amphibians, it turned out that 24 percent of the world's terrestrial vertebrates are threatened with extinction.

As scientists extended the method to other animal groups a larger pattern began to emerge as to the portion that are threatened: 14 percent of dragonflies, 32 of fresh water crabs, 33 of corals.

"There is great variation, but the question that emerges is whether 25 percent is representative of broader biodiversity," Baillie said.

"The idea that one-in-four of the world's species may be threatened with extinction does not seem unreasonable," Baillie said.

"We don't know yet because we have not assessed invertebrate groups and plant groups. But we have begun to do this," he added.

"The first attempt to do this indicated that biodiversity is truly in peril, but we don't see any 700 billion dollar bailout plan on the horizon."

The Congress, organised by International Union for Conservation of Nature (IUCN), brings together more than 8,000 ministers, UN officials, NGOs, scientists and business chiefs to brainstorm on how to brake species loss and steer the world onto a path of sustainable development.

It runs from October 5 to 14.

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